sql: set default initial_retry_backoff_for_read_committed to 2ms

Testing was performed with a 3-node cluster running the following
transaction repeatedly from 32 connections, all trying to lock the same
row:

```sql
BEGIN;
SELECT 1;
SELECT k, v FROM kv WHERE k IN (0) FOR UPDATE;
-- 10 ms pause
UPSERT INTO kv VALUES (0, <random byte>);
COMMIT;
```

This workload was produced using the following commands:

```
cockroach workload init kv \
  --sequential --cycle-length 1 --insert-count 1 \
  --data-loader INSERT --user demo --db defaultdb

cockroach workload run kv \
  --sequential --cycle-length 1 --write-seq S0 \
  --del-percent 0 --read-percent 0 --span-percent 0 \
  --isolation-level read_committed \
  --sel1-writes --sfu-writes --sfu-wait-delay 10ms \
  --user demo --db defaultdb --concurrency 32
```

For this workload, setting `initial_retry_backoff_for_read_committed`
below 500μs caused more than 100 retries, and above 10ms caused
excessively high PMax query latency. 1ms, 2ms, 4ms all performed well.

We pick 2ms as this should be just at or below the typical execution
time of most workloads. 2ms might be too low for longer-running
statements (though still an improvement over 0ms, the current default).

Release note (sql change): Set the default value of session variable
`initial_retry_backoff_for_read_committed` to 2ms. Testing has shown
that some high-contention workloads running under Read Committed
isolation benefit from exponential backoff. 2ms might be too quick of an
initial backoff for longer-running statements, but setting this much
higher than the normal duration of execution will cause excessive delay.

Author: michae2

pkg/sql/logictest/testdata/logic_test/information_schema

@@ -4011,7 +4011,7 @@ idle_in_transaction_session_timeout                        0
 idle_session_timeout                                       0
 index_join_streamer_batch_size                             8.0 MiB
 index_recommendations_enabled                              off
-initial_retry_backoff_for_read_committed                   0
+initial_retry_backoff_for_read_committed                   2
 inject_retry_errors_enabled                                off
 inject_retry_errors_on_commit_enabled                      off
 integer_datetimes                                          on

pkg/sql/logictest/testdata/logic_test/pg_catalog

@@ -3005,7 +3005,7 @@ idle_in_transaction_session_timeout                        0                   N
 idle_session_timeout                                       0                   NULL      NULL        NULL        string
 index_join_streamer_batch_size                             8.0 MiB             NULL      NULL        NULL        string
 index_recommendations_enabled                              off                 NULL      NULL        NULL        string
-initial_retry_backoff_for_read_committed                   0                   NULL      NULL        NULL        string
+initial_retry_backoff_for_read_committed                   2                   NULL      NULL        NULL        string
 inject_retry_errors_enabled                                off                 NULL      NULL        NULL        string
 inject_retry_errors_on_commit_enabled                      off                 NULL      NULL        NULL        string
 integer_datetimes                                          on                  NULL      NULL        NULL        string
@@ -3236,7 +3236,7 @@ idle_in_transaction_session_timeout                        0                   N
 idle_session_timeout                                       0                   NULL  user     NULL      0s                  0s
 index_join_streamer_batch_size                             8.0 MiB             NULL  user     NULL      8.0 MiB             8.0 MiB
 index_recommendations_enabled                              off                 NULL  user     NULL      on                  false
-initial_retry_backoff_for_read_committed                   0                   NULL  user     NULL      0s                  0s
+initial_retry_backoff_for_read_committed                   2                   NULL  user     NULL      2ms                 2ms
 inject_retry_errors_enabled                                off                 NULL  user     NULL      off                 off
 inject_retry_errors_on_commit_enabled                      off                 NULL  user     NULL      off                 off
 integer_datetimes                                          on                  NULL  user     NULL      on                  on

pkg/sql/logictest/testdata/logic_test/show_source

@@ -119,7 +119,7 @@ idle_in_transaction_session_timeout                        0
 idle_session_timeout                                       0
 index_join_streamer_batch_size                             8.0 MiB
 index_recommendations_enabled                              off
-initial_retry_backoff_for_read_committed                   0
+initial_retry_backoff_for_read_committed                   2
 inject_retry_errors_enabled                                off
 inject_retry_errors_on_commit_enabled                      off
 integer_datetimes                                          on

pkg/sql/opt/exec/explain/output_test.go

@@ -362,6 +362,8 @@ func TestRetryFields(t *testing.T) {
 	sqlDB := sqlutils.MakeSQLRunner(conn)
 	sqlDB.Exec(t, "CREATE SEQUENCE s")
 	sqlDB.Exec(t, "CREATE TABLE a (a INT)")
+	// Speed up retries.
+	sqlDB.Exec(t, "SET initial_retry_backoff_for_read_committed = '1us'")
 
 	retryCountRE := regexp.MustCompile(`number of transaction retries: (\d+)`)
 	retryTimeRE := regexp.MustCompile(`time spent retrying the transaction: ([\d\.]+)[µsm]+`)

pkg/sql/vars.go

@@ -4060,7 +4060,7 @@ var varGen = map[string]sessionVar{
 			return strconv.FormatInt(ms, 10), nil
 		},
 		GlobalDefault: func(sv *settings.Values) string {
-			return "0s"
+			return "2ms"
 		},
 	},
 }